1. Field of the Invention
The present invention relates to a sheet screw, and more particularly to an improvement in a thread-forming or self-tapping screw which is especially designed for screwing into a metallic sheet, a thin metallic plate coated with a synthetic resin film or the like sheet member.
2. Prior Art
In fastening a screw into a thin metallic sheet member, a certain conventional method is known wherein a screw is inserted with its threaded shank through a previously drilled hole of slightly greater diameter than the external thread diameter of the screw shank and is then screwed up tightly onto a nut provided at the reverse side of the sheet member for holding the inserted screw. A further method is also known wherein a sheet member is at first formed with a small pilot hole by means of drilling or by means of process, this hole is in the next stage forcibly enlarged by inserting a conical push-out punch or the like thereinto in order to form an integral socket of substantially cylindrical shape that protrudes outwardly from the opening of the hole at the reverse side of the sheet member, and then said hole as well as the protruding socket are internally threaded with a tapping device for facilitating engagement with a mating external thread of a screw. However, such conventional methods are extremely uneconomical. Thus, there have recently been proposed several types of self-tapping sheet screws which are applicable merely to a previously drilled hole of smaller diameter than the external thread diameter of the screw shank. However, such types of screws still waste time and are expensive to use, since they require at least two separate steps, viz. first a drilling operation and then a screwing operation.
Thus, more recently, several different types of self-tapping sheet screws have been proposed which are provided at one shank end with a tapered work-entering portion which has sharp cutting edges extending respectively along an entire tapered surface of said work-entering portion to terminate in a pointed end, whereby a sheet is pierced to form an initial pilot aperture of small diameter, which is, in turn, forcibly enlarged and deformed to form a substantially annular protrusion extending outwardly from the circular edges of the aperture on the opposite side of the sheet, as said work-entering portion intrudes thereinto. Although this type of prior art screw is convenient to use in a sense, since it permits fastening in a single screwing operation, it still has many disadvantages, the first and most significant being that the sheet is not only easily cracked around the vicinity of the formed aperture during the forcible intrusion of said tapered workentering portion, but also suffers from early propagation of fatigue cracks. Further, the initial development of the cracks invites the second disadvantage that the protrusion formed in the sheet material is not sufficient in height, so that the mating threads to be formed on the internal face of the protrusion are not sufficient in number to firmly support the screw shank to be received therein. The third disadvantage is that the aperture is not only excessively enlarged, but the peripheral wall of the aperture is also partially reduced in thickness due to the tearing or scratching-off function of the cutting edges of said tapered portion, so that the desired securing force is not obtained. This third disadvantage is especially important when screwing into aluminum plate or other soft sheet material or into a very thin tinned sheet-iron. The fourth disadvantage is that the screw is likely to advance into the sheet at an angle due to the presence of the cracks caused by the forcible intrusion of the work-entering portion as well as due to the probable lack of uniformity in the thickness surrounding the aperture caused by scratching-off of the sharp cutting edges of the work-entering portion.